1. Field of the Invention
The present invention relates to an overhead cable designed to reduce the wind load and to an overhead cable designed to reduce the wind load and noise.
2. Description of the Related Art
As an overhead cable of the related art, an overhead cable In which an outermost layer Is formed by twisted segment strands having an approximately trapezoidal cross-section and in which a plurality of spiral grooves are provided at spatial Intervals in a circumferential direction on the outermost layer in order to reduce the wind load is disclosed In the specification of U.S. Pat. No. 5,711,143. However, in the overhead cable disclosed In this U.S. patent specification, since the cross-sectional shape of the spiral grooves is that of an arc, though there is an excellent effect in reducing the wind load when the wind speed is low compared with an overhead cable having a plain and smooth surface where no such measures are taken, the rate of reduction of the drag coefficient (wind load) is not sufficient when the wind speed is a high 30 m/s or more. Therefore, there has been a disadvantage that the effect of reducing wind load has not been sufficient in such a region.
An overhead cable in which thick twisted segment strands are provided between thin segment strands having an approximately trapezoidal cross-section to provide spiral-shaped projections on the surface of the outermost layer in order to reduce noise is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 8-273439. However, though the overhead cable disclosed in this publication has an excellent effect in reducing wind noise etc., the drag coefficient is large and therefore the wind load becomes large. Therefore, there has been a disadvantage that the effect in reducing the wind load has been not sufficient.
Furthermore, according to the results of wind tunnel tests so far, when making the surface of a cable as smooth as possible and providing a plurality of spiral grooves of a certain shape in order to obtain the effect of reduction of the wind load, with a cable of the related art in which the outermost layer is comprised of approximately trapezoidal cross-section segment strands, the preformed twisted strands will spring back due to their residual elasticity making it impossible to obtain a good cable surface. Further, since the sides of the trapezoidal segment strands are straight, the strands slip in the diametrical direction when twisting the segment strands to form the outermost layer and therefore the strands will easily end up sticking out or becoming slack making it impossible to obtain a smooth cable surface and obtain the expected effect of reduction of the wind load.
Also, with a cable in which the outermost layer is formed by thin and thick twisted segment strands as explained above, when the cable is passed through a plurality of pulleys for laying out on steel towers, it is repeatedly bent and as a result the segment strands forming the outermost layer slip and end up sticking out or becoming slack causing the problem of an impairment of the effect of reduction of the wind load.
Furthermore, while the twisted cable of the related art was formed by twisting together differently shaped strands which were respectively drawn in advance and shaped by rolling right before being twisted and then pressing the assembly from the outside by a die etc. to shape it. Since there was nothing controlling the positions of the strands after being passed through the die etc., sometimes step differences arose between strands or the strands would become slanted thereby having an unfavorable effect on the cable characteristics and leading to fluctuations in the outer diameter.
European Patent Publication No. 0379853 discloses an overhead cable formed by twisting together segment strands wherein the segments forming the outermost layer are given an approximate S-shaped cross-section. However, this overhead cable is configured with a projecting portion provided at a side of one segment strand of two adjacent segment strands joined with a notched portion provided on the facing side of the other segment strand, therefore while the freedom of movement is restricted to some extent compared with a straight side, there were the problems at the time of laying the cable that the surface of the cable would still easily deform, the segment strands would slip and end up sticking out or becoming slack, and the effect of reduction of the wind load would be impaired. Further, with the overhead cable disclosed in this publication, there was the problem that the wind noise became considerably strong because the surface of the outermost layer was smooth.
Further, in the above overhead cables of the related art, since there was a large length of contact at the adjoining contact portions of the segment strands, there were the problems of a large contact friction, poor flexibility and therefore difficult handling resulting in excessive stress applied to the strands in the process of twisting and drawing and thereby causing linear scratches, burrs, and the like.